Earth's History
The Earth's History domain covers relative and absolute dating, fossils and correlation, the geologic time scale, and major events in the history of life and of New York State. Questions ask you to order events from a cross-section, calculate ages from half-lives, and recognize which dating method fits a sample.
Relative dating principles
- Superposition: in undisturbed strata, older rocks lie below younger.
- Original horizontality and lateral continuity: sedimentary layers form flat and extend until they pinch out.
- Cross-cutting relationships: faults, dikes, and intrusions are younger than the rocks they cut.
- Inclusions are older than the rock containing them; baked contacts show an intrusion is younger.
- Unconformities represent erosion or nondeposition-missing time.
Absolute dating
- Half-life is constant for an isotope; after n half-lives, the parent fraction is (1/2)^n.
- Carbon-14 dates organic remains up to roughly 50,000 years; uranium-238 dates ancient igneous rocks.
- A closed system is required; heating or weathering can reset or corrupt a date.
- Index fossils are widespread, short-lived species used to correlate strata.
Geologic time and life
- Precambrian time spans about 88% of Earth's 4.6-billion-year history.
- The Cambrian explosion (~541 Ma) marks a rapid diversification of animals.
- The Permian-Triassic extinction (~252 Ma) was the largest; the Cretaceous-Paleogene extinction (~66 Ma) ended the dinosaurs.
- Pleistocene glaciation shaped New York's Finger Lakes, moraines, and erratic boulders.
Worked example
A rock contains 25% of its original parent isotope. Because 25% = (1/2)², two half-lives have passed. If the half-life is 1.25 billion years, the rock is 2 × 1.25 = 2.5 billion years old. If 12.5% remained, three half-lives (3.75 billion years) would have elapsed.
Traps that cost marks
- Believing half-life changes with temperature or pressure.
- Using carbon-14 on igneous rock or on samples older than ~50,000 years.
- Reversing cross-cutting order (a dike is younger than the beds it cuts).
- Treating an unconformity as a continuous record.
- Forgetting that index fossils must have a wide geographic range and a short time range.